1. Field Of The Invention
The present invention relates to a joint seal for use in high pressure and high temperature conditions, and more particularly to such a seal system fabricated out of a metal alloy having shape memory characteristics in combination with a compliant seal, which permits seating of the seal system after the mechanical makeup or connection has been fully completed.
2. Description Of The Prior Art
The exploration for and extraction of natural gas has in some instances presented problems not commonly associated with other types of oil and gas wells. Extremely high temperatures and especially high pressures associated with the deeper gas wells present various problems that could not have even been imagined by the earlier drillers of the more shallow oil and gas wells. However, with the exhaustion of the more easily recoverable oil and gas, it has become necessary to drill deeper into the earth, seeking replacement supplies of these vital commodities. The hostile conditions present at these greater depths are constantly challenging metallurgists and petroleum engineers to develop new systems capable of performing under these conditions.
The drilling method historically employed by the petroleum industry for the drilling of exploratory and producing wells utilizes a drill bit attached to a rotating string of drilling pipe. Rotation is induced by a rotary table located on the surface, which is engaged with the string of pipe by a special, uppermost section of drilling pipe, or Kelly. As the drill bit bores into the earth, at rates often averaging about five feet per hour (1.5 meters per hour) in deep holes, additional sections of drilling pipe are added from the top. During both the drilling and production, it is vital that connections between the individual lengths of drilling pipe as well as other types of well tubulars, including various kinds of casing, lining, and tubing, also provide gas and fluid-tight seals. Such sealing is necessary to ensure containment during drilling and production of the hazardous high pressures existing in deep gas wells. The difficulties in maintaining these individual seals is made infinitely more complicated by high and widely varying magnitudes of temperature and pressure, and frequently, highly corrosive environments.
In providing pressure-tight joints at ultra-high pressures, whether or not in the oil industry, it is typically not possible to utilize compliant seal material as the primary seal. Under high differential pressures, such material will structurally fail and extrude out through any available opening. For this reason, metal-to-metal seals are provided as the primary seal. Metal seals are created both through the provision of corresponding planar sealing surfaces and through specially designed, tapered threads. Use of a compliant sealing material in these circumstances is limited to the formation of secondary seals, which are created in protected areas located between the metal-to-metal seals. Such metal-to-metal seals have been widely used in the past over varying temperature and pressure conditions, however at increasing temperatures and pressures, These concepts have proven to be unreliable.
Most sealing systems utilize either the application of a preload or the creation of an interference, either of which energizes the seal and creates sealing stresses. Because metals have relatively high modulus and low compliance, relatively high stresses and preloads are required with metal seal systems. Furthermore, stretching of the connection due to weight of the tubular, internal pressure, flexing of the tubular, and differential thermal expansion can cause unloading of the preload, and thus seal failure. To prevent this from occurring, additional, compensating preloading is required. After the sealing surfaces have been brought together during the mechanical formation of the connection, it is necessary to further tighten the members, pressing the sealing surfaces tightly together to preload them sufficiently to form a reliable seal. All of these additional components of preloading increase the stresses and frictional forces in the connection.
Since the mating or sealing surfaces are also part of the mechanical joint system, these surfaces are subjected to rotational sliding, under load, during the mechanical makeup of the joint. In fact, the seals are formed as a result of the two sealing surfaces sliding past one another under increasing stresses. Such a sealing process inevitably creates the potential for damage to the sealing surfaces because of the interference fit. Minimization of the amount of damage caused thereby requires creation of extremely tight tolerances and precise interference fits, which are both expensive to obtain and excessively vulnerable to damage.
The precise interference fits also prevent the effective and reliable use of resilient or compliant materials as the primary seal. Such materials are unable to withstand the strain placed upon them as the tightly confining mating surfaces rotate against them. Bunching, rolling, and/or galling is highly probable, and such destructive deformation is catastrophic to the formation of a viable seal. Even should the material somehow escape damage during the mechanical makeup, it is inevitable that gaps will exist within the threads or between sealing surfaces, and at high pressures, the sealing material will fail by extruding out of its seat and into these gaps. For these reasons, compliant materials, which are the most desirable seal materials, are seldom used in forming such high pressure seals, and even where used, metal-to-metal seals provide the primary sealing means.